Spectral types of planetary host star candidates from OGLE III

Context. The Optical Gravitational Lensing Experiment project has recently provided the OGLE III list of low-luminosity object transits from campaigns #3 and #4, reporting 40 new objects exhibiting the low-amplitude photometric eclipses expected for exoplanets. Compared to previous OGLE targets, these OGLE III candidates have been more restrictively selected and may contain low-mass planets. Aims. We have secured follow-up low-resolution spectroscopy for 28 candidates out of this list (and one from the OGLE Carina fields) to obtain an independent characterization of the pri mary stars by spectral classification and thus better constr ain the parameters of their companions. Methods. We fed the constraints from these results back into an improved light curve solution. Together with the radius ratios fro m the transit measurements, we derived the radii of the low-luminosity companions. This allows us to examine the possible sub-stellar nature of these objects. Results. Sixteen of the companions can be clearly identified as low-ma ss stars orbiting a main sequence primary, while 10 more objects are likely to have red giant primaries and therefore also host a stellar companion; 3 possibly have a sub-stellar nature (R� 0.15 R⊙). Conclusions. The planetary nature of these objects should therefore be confirmed by dynamical mass determinations.